Mobile wireless communication systems, for example cellular telephony or private mobile radio communication systems, typically provide for radio telecommunication links to be arranged between a plurality of user or subscriber terminals, often termed ‘mobile stations’, MSs, via a system infrastructure including fixed installations including one or more base transceiver stations (BTSs).
Mobile communication systems typically operate according to a set of industry standards or protocols. An example of such standards is the TETRA (TErrestrial Trunked Radio) standards, which have been defined by the European Telecommunications Standards Institute (ETSI). A system that operates according to TETRA standards is known as a TETRA system. TETRA systems are primarily designed for use by professional radio users, such as the emergency services.
TETRA systems operating according to the existing standards are used primarily for voice communication and provide limited slow data communication. A second generation of TETRA standards is being developed. This second generation is aimed at providing high speed data communication, for example for fast accessing of police databases, and for transfer of pictures, image and video data and the like. The existing generation of TETRA standards is referred to as ‘TETRA 1’ standards and the new standards are referred to as ‘TETRA 2’ standards, and in one form are known as ‘TEDS’ (‘TETRA Enhanced Data Services’) standards.
The TETRA systems are just one example of an increasing demand for multi-band/multi-mode wireless devices in the field of wireless communications.
In order to support multi-band/multi-mode operation, wireless devices need to incorporate more complex receiver architectures to enable the various multi-band/multi-mode signals to be received and processed utilizing a single receiver architecture, thereby avoiding the need to provide multiple receiver circuits respectively dedicated to support each multi-band/multi-mode operation.
In wireless communications, the dual conversion receiver is one of the most well-known receiver architectures. The dual conversion receiver first converts a received RF signal to a first intermediate frequency (IF1) and then converts the IF1 signal to a second intermediate frequency (IF2). In a dual-conversion receiver, bandpass IF filters are used at numerous stages to filter out unwanted/interfering mixing products or interfering adjacent channel signals.
However, as a consequence of having distinct frequency stages, dual-conversion receivers require more, and therefore more costly, components and circuit space. Hence, there is a need for a cost-effective receiver that circumvents the overhead associated with a traditional dual conversion receiver.
An alternative to the dual conversion receiver is a very low intermediate frequency (VLIF) receiver. The VLIF receiver converts the received RF frequency to a very low (intermediate) frequency signal that can be digitally filtered before processing.
A yet further alternative to the dual conversion receiver is a direct-conversion receiver (DCR) architecture 100, sometimes referred to as a zero IF receiver, as shown in FIG. 1. Here, a received radio frequency signal 105 is input to an RF bandpass filter or circuit 110 arranged to remove out-of-band (unwanted) signals. The filtered output is input to a low noise amplifier (LNA) 115 to amplify the desired signal to allow further signal processing. The differential outputs 120, 125 of the LNA 115 are respectively input to quadrature (‘I’ and ‘Q’) frequency down conversion circuits. Here, a local oscillator (LO) 140 operating at the received RF frequency provides a down-conversion RF signal to mixers 130, 135, one of which is passed through a 90 degree phase shifter 145 to provide the quadrature down-conversion. In this manner, the amplified received signal is down-converted to a baseband signal by mixing the signal with a reference RF signal at the same frequency. The respective baseband (‘I’ and ‘Q’) signals are then amplified in baseband amplifiers 150, 155 and the baseband amplified signals filtered in low pass filters 160, 165 to respectively produce baseband ‘I’ data 170 and baseband ‘Q’ data 175.
In this manner, the DCR receiver architecture 100 directly converts the received RF frequency 105 to baseband signals 170, 175 that can be directly processed, without a need to utilize intermediate frequency circuits. Thus, this architecture represents a fully integrated receiver that eliminates the need for both IF and image-reject filtering, and requires only a single local oscillator (LO) source.
Unfortunately, there are some limitations regarding architectures, such as DCR, that adversely impact the accurate reception and processing of the desired signal. These limitations include:
(i) dynamic dc (direct current) offsets that are generated primarily from the self-mixing of RF or LO signals through undesirable leakage paths within the RF front-end circuitry;
(ii) 1/ƒ noise at the RF front-end, which affects receiver sensitivity for narrow-band systems; and
(iii) second-order intermodulation products (IP2) inherently created due to linearity performance limitations of the receiver's components. These IP2 products introduce undesirable spectral components at baseband, thereby degrading receiver sensitivity.
Whilst (i) and (ii) have existing solutions that meet most system requirements, the effect of IP2 products in a DCR receiver still remains the most challenging obstacle to overcome, particularly for systems requiring a high adjacent channel interference (blocking) performance. The best reported mixer IP2 performance of +60 dBm, using fully differential circuitry alone, is typically inadequate. This level of performance was reported in IEEE Journal of Solid State Circuits paper, vol. 35 No. 12 of December 200, titled “A high IIP2 downconversion mixer using dynamic matching”,
Hence, in order to reduce the effects of IP2, particularly in DCR receiver architecture, it is known to use chopping mixers (also known as dynamically matching mixers) to improve receiver performance. The mixer stage IP2 may be improved from +60 dBm to +74 dBm.
However, supporting use of such mixers creates an increase in receiver current drain of the order of 30%. This is undesirable in a battery-powered device, such as a wireless communication unit.
Thus, it is desirable to eliminate or reduce current drain used in a direct conversion receiver having at least one chopping mixer stage to enable the direct conversion receiver to operate with a receiver current drain similar to that of a dual conversion receiver.
Hence, a need exists for an improved receiver architecture, a wireless communication unit comprising such a receiver architecture and a method of frequency conversion that may alleviate one or more of the aforementioned problems.